Re: [PATCH v2] sched/fair: check for idle core

[Julia Lawall]

On Mon, 25 Jan 2021, Vincent Guittot wrote:

> On Mon, 25 Jan 2021 at 10:20, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
> >
> >
> >
> > On Mon, 25 Jan 2021, Mel Gorman wrote:
> >
> > > On Sun, Jan 24, 2021 at 09:38:14PM +0100, Julia Lawall wrote:
> > > >
> > > >
> > > > On Tue, 27 Oct 2020, Mel Gorman wrote:
> > > >
> > > > > On Thu, Oct 22, 2020 at 03:15:50PM +0200, Julia Lawall wrote:
> > > > > > Fixes: 11f10e5420f6 ("sched/fair: Use load instead of runnable load in wakeup path")
> > > > > > Signed-off-by: Julia Lawall <Julia.Lawall@xxxxxxxx>
> > > > > > Reviewed-by Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > > > >
> > > > >
> > > > > While not a universal win, it was mostly a win or neutral. In few cases
> > > > > where there was a problem, one benchmark I'm a bit suspicious of generally
> > > > > as occasionally it generates bad results for unknown and unpredictable
> > > > > reasons. In another, it was very machine specific and the differences
> > > > > were small in absolte time rather than relative time. Other tests on the
> > > > > same machine were fine so overall;
> > > > >
> > > > > Acked-by: Mel Gorman <mgorman@xxxxxxx>
> > > >
> > > > Recently, we have been testing the phoronix multicore benchmarks.  On v5.9
> > > > with this patch, the preparation time of phoronix slows down, from ~23
> > > > seconds to ~28 seconds.  In v5.11-rc4, we see 29 seconds.  It's not yet
> > > > clear what causes the problem.  But perhaps the patch should be removed
> > > > from v5.11, until the problem is understood.
> > > >
> > > > commit d8fcb81f1acf651a0e50eacecca43d0524984f87
> > > >
> > >
> > > I'm not 100% convinved given that it was a mix of wins and losses. In
> > > the wakup path in general, universal wins almost never happen. It's not
> > > 100% clear from your mail what happens during the preparation patch. If
> > > it included time to download the benchmarks and install then it would be
> > > inherently variable due to network time (if download) or cache hotness
> > > (if installing/compiling). While preparation time can be interesting --
> > > for example, if preparation involves reading a lot of files from disk,
> > > it's not universally interesting when it's not the critical phase of a
> > > benchmark.
> >
> > The benchmark is completely downloaded prior to the runs.  There seems to
> > be some perturbation to the activation of containerd.  Normally it is
> > even:  *   *   *   *
>
> Does it impact the benchmark results too or only the preparation prior
> to running the benchmark ?

Looking at a few of the benchmarks, there is no clear pattern which is
better.  But there is not a big degradation, like from 23 to 28 seconds
for the preparation time.  I will report back when we figure out more.

julia

>
> >
> > and with the patch it becomes more like: *     **     **
> >
> > That is every other one is on time, and every other one is late.
> >
> > But I don't know why this happens.
> >
> > julia
> >
> > >
> > > I think it would be better to wait until the problem is fully understood
> > > to see if it's a timing artifact (e.g. a race between when prev_cpu is
> > > observed to be idle and when it is busy).
>
> I agree that a better understanding of what is happening is necessary
> before any changes
>
> > >
> > > --
> > > Mel Gorman
> > > SUSE Labs
> > >
>